Unique Spawning Movements and Behavior of Adfluvial Bull Trout in a Protected Watershed

Spawning season movements of adfluvial bull trout (Salvelinus confluentus) were evaluated using acoustic telemetry transmitters and PIT (passive integrated transponder) data during spawning years 2006 to 2010 in Chester Morse Lake, Washington. Of the 68 monitored adult bull trout, 56 were detected making unique spawning movements during the study. A great majority of all tagged bull trout made multiple trips between the reservoir and mainstem riverine habitat during a single spawning year, usually during dawn and dusk periods. A morning peak in daily movement was observed near sunrise, while the evening peak occurred near dusk. Many tagged fish made trips into the river during dusk hours, remained in the river at night, and returned to the reservoir during morning hours. This pattern was repeated over periods of several days, or in some cases over a period of several months. The unique tag identifying each fish allowed us to quantify the number of trips made between the river and reservoir over multiple spawning seasons. While some fish made only a single spawning entry into the river, several fish were documented making more than 30 trips between the reservoir and the river during a single spawning season. This study demonstrates that the spawning behavior of adfluvial bull trout is complex and that protecting essential habitat for this species requires a clear understanding of their movement patterns. By making multiple trips between the reservoir and river, bull trout in the Chester Morse Lake population likely reduce the risk to redds from any single environmental factor or event, such as scouring flows, over a greater temporal and spatial range. In addition, genetic variability may be increased in the overall population as females likely spawn with different males on subsequent trips. The study also demonstrates that the ability to monitor an individual’s movement over a 24-hour period is very important when documenting, and subsequently evaluating, spawning behavior, as highly significant activity may occur during evening hours when traditional telemetry methods do not actively monitor fish movement.